The invention relates to a process and to a plant for low-pressure casting of a metallic composition in the liquid state in a mold comprising a ceramic shell, one face of which delimits a casting space, and a mass of binder-free sand in contact with the other face of the shell, the mold being placed in a box having at its base an access joined to the casting space and connected via a liquid-metal feed pipe to a source of liquid metal located underneath, for example a casting furnace.
Already known are casting processes and plans of this type, in which the source of liquid metal, such as a casting furnace, is brought into communication with a source of gas under pressure in such a way that the liquid metal contained in the source of liquid metal is pressurized to a first pressure value by means of which the liquid metal is brought into and maintained in the region of the access of the box for this liquid metal.
When a casting is desired to be cast, by introducing liquid metal into the mold, the level of the liquid metal is raised above the threshold of the mold where it was, by increasing the pressure up to a second value slightly greater than the value necessary for making the liquid metal fill the casting space, so as to have a safety margin and to be certain that all the intricacies of the mold, even the highest ones, are filled with liquid metal, and the pressure is maintained at this second value for a time long enough to allow a rest phase, and a low metallostatic pressure on the walls of the ceramic shell. The pressure is then increased further to a third value so as to fill up any shrinkage cavities or holes, and the pressure is maintained at this third value for a time allowing the desired solidification of the metal contained in the mold to take place, after which the pressure is reduced so that the level of the unused liquid metal goes down to approximately the initial level, and this pressure is maintained for the time necessary, for example, to remove the mold, de-mold the casting and put a new mold in place to allow manufacture of another casting.
However, the ceramic shells of these molds generally have an extremely small thickness and thus a degree of flexibility which, when they are subjected to the pressure of the liquid metal, makes them tend to suffer deformations which impair their dimensional stability. In order to overcome any dimensional instability that might occur in the case of the casting process described above, it has been proposed to keep the mass of sand in contact with the shell at an underpressure, so as to provide the grains of sand with dynamic stability and thus to ensure that the shell is dimensionally stable. For this purpose, the box is provided with a second access joined to the space containing the mass of sand and connected via a suction pipe to a device for creating an underpressure.
However, creating an underpressure in the mass of sand in this way has the drawback of disturbing the rise and the proper distribution of the liquid metal in the casting space by means of the porosity of the ceramic (xe2x80x9cvacuumxe2x80x9d filling), which in turn causes too rapid filling, producing defects in the castings.
The object of the invention is to overcome these drawbacks and relates for this purpose to a process for low-pressure casting in a mold comprising a ceramic shell, one face of which delimits a casting space, and a space containing a mass of binder-free sand in contact with the other face of the shell, the mold being placed in a box having at its base an access joined to the casting space and connected to a source of liquid metal placed underneath, in which process the liquid metal in the source is pressurized to a first pressure value suitable for bringing liquid metal into the region of the access, the pressure is increased to a second value slightly greater than the value necessary for filling the casting space and this second value is maintained for a first predetermined time, the pressure is then increased to a third value and this third value is maintained for a second predetermined time, the pressure is then reduced so that the level of the liquid metal goes down, approximately into the region of the access or below, characterized in that, approximately when the casting space becomes entirely filled with liquid metal, the pressure in the space containing the mass of sand is reduced to below atmospheric pressure to a predetermined underpressure value, this underpressure value is maintained at least until a solidified skin is obtained against the shell and the pressure in the space containing the mass of sand is brought back up to approximately atmospheric pressure.
The process according to the invention may also have one or more of the following characteristics:
the duration of the phase during which the pressure in the space containing the mass of sand is reduced to below atmospheric pressure, to a predetermined underpressure value, is shorter than the first predetermined time;
the phase during which the pressure in the space containing the mass of sand is increased, to approximately atmospheric pressure, is brought to its completion before the completion of the phase during which the pressure to which the liquid metal contained in the source of liquid metal is subjected is decreased, in such a way that the level of the liquid metal goes down approximately into the region of the access or below;
the predetermined underpressure value corresponds to an absolute pressure in the range of approximately 0.5 to 0.9 bar;
the first pressure value is about 1.15 bar absolute;
the second pressure value is about 1.4 bar absolute;
the third pressure value is about 1.7 bar absolute.
By virtue of the fact that it is only when the casting space is filled with liquid metal that the pressure in the space containing the mass of sand is reduced, the filling is not disturbed by the underpressure.
The invention also relates to a plant for implementing the above process, which includes a mold comprising a ceramic shell, one face of which delimits a casting space, and a space containing a mass of binder-free sand in contact with the other face of the shell, the mold being placed in a box having at its base an access joined to the casting space and connected to a source of liquid metal located underneath, which plant is characterized in that the box includes, on the inside, a vacuum chamber provided with at least one region of an air-permeable wall and having an access opening fitted with a vacuum connector in order to be connected to a suction device via a solenoid valve controlled by a programmable controller which also controls a device for regulating the pressure in the source of liquid metal.
The plant according to the invention may also be characterized in that the vacuum chamber extends annularly in the box against a base wall and side walls of the box.
By virtue of this plant, the molding process may be very rapidly implemented and accurately controlled with an installation investment that nevertheless remains modest.